A chemical blood gas sensor (a preferred embodiment being referred to as a HEMOMETRIX.TM. sensor) has been developed for monitoring pCO.sub.2, pO.sub.2, and pH in a patient's vascular system. The sensor is disposed at the distal end of an optical fiber and is introduced into a patient's blood stream through a preinserted catheter. To facilitate ease of use and handling of the HEMOMETRIX.TM. sensor, it is packaged in a sterile environment during storage and shipment from the manufacturer to the point of use. This sterile environment comprises a flat tray having a plurality of compartments that hold various components of the sensor and of a delivery device that is connected to the catheter for introduction of the sensor into the patient's bloodstream. The tray is covered with a gas permeable sheet of TYVEK.TM. that resists penetration by bacteria, but allows sterilization of the package and its contents by exposure to ethylene oxide (ETO) gas. U.S. Pat. Nos. 4,863,016 and 5,111,836, owned by the same assignee as the present case, disclose details of the packaging and other related aspects. The disclosure and drawings of U.S. Pat. Nos. 4,863,016 and 5,111,836 are hereby specifically incorporated herein by reference, since many of the components and configuration of the package for the sensor that are disclosed therein directly relate to the present invention.
At the time that the HEMOMETRIX.TM. sensor is packaged, a tonometry chamber in which the sensor is disposed is filled with a buffered hydration solution from a pouch connected to the chamber by plastic tubing, immersing the sensor in the hydration solution. After the tonometry chamber is filled, a "guillotine" valve provided in the tray is actuated, severing and sealing the line, so that the pouch can be discarded. The tray is then sealed in a gas impermeable wrap and remains sealed until the HEMOMETRIX.TM. sensor is to be calibrated, just prior to the sensor's use.
Since it is important to maintain the HEMOMETRIX.TM. sensor and its associated apparatus in a sterile environment, a calibration method and apparatus for calibrating the sensor while it is in the tray and covered by the gas permeable sheet are required. Furthermore, although it is possible to calibrate the sensor at ambient temperature and then apply temperature corrections to the readings of blood gas values obtained with the sensor at the substantially higher temperature of its use in vivo, the results are not entirely satisfactory. Much more accurate measurements can be obtained with the HEMOMETRIX.TM. sensor by calibrating at a plurality of calibration points made at a calibration temperature that is substantially equal to the temperature at which the sensor will actually be used, i.e., at the temperature of the patient's blood, 37.degree. C. Before the sensor is calibrated, it must be immersed in a calibration solution to obtain a first calibration point. Ideally, the calibration solution should then be chemically modified to obtain a second calibration point, to improve the accuracy with which it subsequently monitors the blood gases and pH. No prior art calibration device or method provides the facility to calibrate an in vivo blood gas sensor at the temperature it will be used in the patient and at a plurality of calibration points, while maintaining the sensor in its sterile environment.